Turn CSSParserContext into a class

This change makes it more obvious when a member gets overwritten
(which shouldn't happen but unfortunately does).

It also will help storing the Document into the context to allow
deprecation messages in the CSSParser.

Review URL: https://codereview.chromium.org/84903002

git-svn-id: svn://svn.chromium.org/blink/trunk@162648 bbb929c8-8fbe-4397-9dbb-9b2b20218538